Unfortunately, in such a case of a patched roof deck layers upon layers of roofing membranes need to be completely torn off, deck included.

You’ll need to completely remove your siding and metal cap flashing to bring your membrane 3” minimum above your finished deck surface. Encapsulating your parapet wall in the membrane is highly recommended, up and over.

More importantly, your new deck surface should not lay directly on your new roof surface. This will obstruct water flow to the drain. Raise your new deck off of the roof with rubber pads or strips of the equivalent membrane.